1. Field of the Invention
The present invention relates to a semiconductor device utilizing a silicon carbide (SiC) substrate and its manufacturing method.
2. Description of the Related Art
A silicon carbide semiconductor (hereinafter, abbreviated as SiC) is capable of forming a pn junction and a forbidden band width thereof is wide as compared with other semiconductor such as a Silicon (Si) or a Gallium Arsenide (GaAs). It is reported that the forbidden band width of 3C—(C denotes a cubic system as will be described later) SiC is 2.23 eV (electron Volt), that of 6H—(H denotes a hexagonal system as will be described later) SiC is 2.93 eV, and that of 4H-SiC is 3.26 eV. As is well known, there are trade-off relationships in principle prescribed by the forbidden band width between an on resistance of a power device and a reverse direction withstanding voltage thereof and between the on resistance thereof and a switching frequency (switching speed) thereof. Hence, it is difficult to obtain a high performance exceeding a limit determined by the forbidden band of Si from currently available Si power devices. However, since, if the power device is constituted by SiC with the wide forbidden band width, the above-described trade-off relationships are largely relieved, such a power device that the on resistance, the reverse direction blocking voltage, and the switching speed have remarkably or simultaneously been improved can be realized. Furthermore, since SiC is thermally, chemically, and mechanically stable and is superior in a radiation ray withstanding characteristic, it is expected that SiC can be realized not only as a high frequency device and the power device but also as an environment withstanding characteristic semiconductor device which operates under a strict condition such as a high ambient temperature, an erosion, and radiation ray irradiation.
In a MOS (Metal Oxide Semiconductor) capacitor, SiC power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) to control a large current, and an IGBT (Insulated Gate Bipolar Transistor) to control the large current especially from among SiC devices, it is an important problem to be solved for SiC devices to be put into practice that a contact resistance on a source and a drain (n type polarity) which provides causes of a thermal loss increase and of an operating speed reduction is reduced to a negligible level and highly reliable and high performance gate insulating film and MOS interface characteristic are realized.
A technology to obtain a low contact resistance in SiC singlecrystalline has been proposed. That is to say, the following method has been proposed. After a contact metallic film is formed on SiC through a vacuum deposition, a rapid thermal annealing (RTA) is carried out for several minutes at a high temperature heat process (so-called, a contact annealing) is carried out for several minutes at a high temperature equal to or higher than 950° C. under a vacuum or inactive gas atmosphere to form a reaction layer between SiC and the contact metal which provides a contact electrode. According to a Journal of Applied Physics, 77, page 1317 (1995) authored by J. Crofton et al., n type region of SiC substrate using an Ni (Nickel) film indicates the contact resistance of an extremely low practical level in an order of 10−7 Ωcm2. According to a book authored by J. Crofton called Solid-State Electronics, 41, page 1725 (1997), p type region of SiC substrate using an Al (Aluminum)-Ti (Titanium) alloy film indicates the contact resistance of the extremely low practical level in an order of 10−6 Ωcm2. In addition, in recent times, the low contact resistance in an order of 10−7 Ωcm2 is also obtained in each of n type region and p type region of 4H-SiC substrate using a thin Ni and a Ti—Al laminated layer.